[PATCH RFC 07/29] mm/migrate: rename isolate_movable_page() to isolate_movable_ops_page()

[Alistair Popple]

On Sun, Jun 29, 2025 at 07:28:50PM +0800, Huang, Ying wrote:
> David Hildenbrand <david at redhat.com> writes:
> 
> > On 18.06.25 20:48, Zi Yan wrote:
> >> On 18 Jun 2025, at 14:39, Matthew Wilcox wrote:
> >> 
> >>> On Wed, Jun 18, 2025 at 02:14:15PM -0400, Zi Yan wrote:
> >>>> On 18 Jun 2025, at 13:39, David Hildenbrand wrote:
> >>>>
> >>>>> ... and start moving back to per-page things that will absolutely not be
> >>>>> folio things in the future. Add documentation and a comment that the
> >>>>> remaining folio stuff (lock, refcount) will have to be reworked as well.
> >>>>>
> >>>>> While at it, convert the VM_BUG_ON() into a WARN_ON_ONCE() and handle
> >>>>> it gracefully (relevant with further changes), and convert a
> >>>>> WARN_ON_ONCE() into a VM_WARN_ON_ONCE_PAGE().
> >>>>
> >>>> The reason is that there is no upstream code, which use movable_ops for
> >>>> folios? Is there any fundamental reason preventing movable_ops from
> >>>> being used on folios?
> >>>
> >>> folios either belong to a filesystem or they are anonymous memory, and
> >>> so either the filesystem knows how to migrate them (through its a_ops)
> >>> or the migration code knows how to handle anon folios directly.
> >
> > Right, migration of folios will be handled by migration core.
> >
> >> for device private pages, to support migrating >0 order anon or fs
> >> folios
> >> to device, how should we represent them for devices? if you think folio is
> >> only for anon and fs.
> >
> > I assume they are proper folios, so yes. Just like they are handled
> > today (-> folios)

Yes, they should be proper folios.

> > I was asking a related question at LSF/MM in Alistair's session: are
> > we sure these things will be folios even before they are assigned to a
> > filesystem? I recall the answer was "yes".
> >
> > So we don't (and will not) support movable_ops for folios.
> 
> Is it possible to use some device specific callbacks (DMA?) to copy
> from/to the device private folios (or pages) to/from the normal
> file/anon folios in the future?

I guess we could put such callbacks on the folio->pgmap, but I'm not sure why
we would want to. Currently all migration to/from device private (or coherent)
folios is managed by the device, which is one of the features of ZONE_DEVICE.
Did you have some particular reason/idea for why we might want to do this?

> ---
> Best Regards,
> Huang, Ying